This invention relates to semiconductor optical devices. More specifically it relates to a novel tapered rib waveguide structure useful in coupling the output of the semiconductor optical device into and out of an optical fiber. Even more specifically, it relates to a device that achieves two dimensional expansion or contraction of the output optical mode of single-transverse-mode semiconductor waveguide modulators and lasers.
High performance optoelectronic systems using optical fiber signal routing require low-cost and low-loss coupling between single-mode fibers and active or passive semiconductor waveguide devices and circuits. However, the 1-3 .mu.m elliptical modal spot of typical semiconductor waveguides is poorly matched to the 8-9 .mu.m circular modal spot of conventional single-mode optical fibers. This modal mismatch results in a 7-10 dB insertion loss when directly coupling light between these single-mode fibers and semiconductor waveguides. Non-integrated solutions that improve this coupling efficiency often do so at the cost of tight alignment requirements and are thus not well suited for low-cost package using passive alignment systems. Tapered waveguide transitions offer a monolithically-integrated means by which efficient coupling can be achieved with relaxed alignment requirements. Most of these approaches, however, require complex growth or processing steps such as multiple etch-regrowth sequences or lithographic patterning of extremely small radius waveguide tips in order to achieve the desired coupler performance.
One reference of interest is U.S. Pat. No. 5,574,742 for a "Tapered Beam Expander Waveguide Integrated with a Diode Laser." Therein the structure analogous to the tapered rib in the present invention, therein called the first semiconductor waveguiding layer is etched in a multi-step process to form a very sharp (less than 500 Angstroms radius) edge. The light emanating from this edge then is coupled into the optical fiber. The process to form the edge is complicated, the sharpness of the edge is essential to prevent scattering of light, and the height of the structure is critical, given that an etch stop is necessary below the first semiconductor waveguiding layer. This structure does not force the light in this layer down into a lower layer, as is done in the present invention.
There remains in the art an unmet need for an improved waveguide coupler structure that can be made easily and yet couple the light between a semiconductor optical device, such as an optical modulator or a laser, and an optical fiber with very high efficiency.